The present invention relates to methods and compositions for inhibiting polymerization of diene monomers, and more particularly relates, in one embodiment, to methods and compositions for inhibiting the polymerization of butadiene which gives rise to popcorn polymer growth.
In the production of an olefin, such as a diene, the so-called popcorn polymer having a porous, three-dimensional structure occurs frequently and undesirably in process apparatus due to the unintentional polymerization of the olefin in refining, distillation and recovery or during recovery of the monomer after termination of intentional polymerization, such as during the production of synthetic rubber, in particular styrene-butadiene rubber. Popcorn polymer occurs both in the gaseous phase and the liquid phase. It is more likely to occur when the concentration of the olefin monomer is high and the temperature is high. A minute amount of oxygen, such as may come from a peroxide, may act as an initiator for the polymerization reaction. Iron rust, if present, accelerates the reaction of popcorn polymerization to a great extent.
Numerous olefin monomers such as styrene, xcex1-methyl styrene, acrylic acid and esters thereof, vinyl acetate, acrylonitrile, acrylamide, methacrylamide, etc. and such dienes (diolefins) as 1,3-butadiene, isoprene, and chloroprene, upon reaching refining devices during production and recovery, are exposed to certain conditions such as high temperature, high monomer concentration, coexistence of vapor and liquid phase, humidity, trace oxygen and iron rust which are highly conducive to the occurrence of popcorn polymerization. Fouling of equipment can even occur when unsaturated compounds in petroleum or its derivatives undesirably polymerize.
The popcorn polymer is disposed to forming xe2x80x9cseedsxe2x80x9d which may continue to propagate until the monomer ceases to exist. Because of this phenomenon, minute particles of the popcorn polymer rapidly grow into large lumps of polymer. The popcorn polymer therefor adheres to and defiles the heat-exchanger, distillation tower, and piping installed within the system for refining and recovering the produced olefin and deteriorates the efficiency of the refining operation. It often clogs the apparatus and piping. In an extreme case, the mechanical pressure generated during the propagation of the polymer may deform and fracture the apparatus.
The reason for the rapid propagation of the popcorn polymer is that, as the polymer forms, radically active sites are newly formed inside the polymer and the polymer attains growth from the newly formed radically active sites. These radically active sites have a surprisingly long life. When the polymer is exposed to the ambient air during suspension of the operation of the apparatus, and brought into contact with the monomer as a result of the resumption of operation, it again starts growing and propagating from the active sites present.
The popcorn polymer is of such a quality that it is insoluble in all solvents and defies removal by heating. For the removal of the intractable popcorn polymer, the apparatus must be disassembled and mechanically cleaned. Temporary suspension of use of the apparatus and the cleaning thereof bring about an immense economic loss.
Numerous inhibitors have been proposed for the purpose of inhibiting popcorn polymerization. Examples include, but are not necessarily limited to, nitrates, nitrogen oxides, nitroso compounds, alkyl phenols, aromatic amines, hydroxylamines, etc. For these inhibitors to be effectively used, they must be continuously injected into the apparatus during its operation.
The compounds called alkyl-substituted di-nitro-phenols and nitroso-phenols found widespread use in the styrene industry. However, because such compounds also functioned as insecticides or were dangerous to handle, their use has been discouraged by environmental and government agencies.
Recently, a new class of compounds called stable free radicals is being investigated to replace the nitrophenol products. Although stable free radicals are effective against monomer polymerization, their current cost makes them unattractive. It would be desirable if a composition and method could be devised to overcome some of the problems in using the stable free radical polymerization inhibitors.
It will also be appreciated that not all polymerization inhibitors are effective to inhibit the undesired polymerization of all olefins, particularly dienes. Thus, it cannot be assumed that a polymerization inhibitor useful for mono-olefins is effective to inhibit undesired polymerization of dienes. Nor can it be assumed that a compound or composition effective in inhibiting polymerization of one diene, is necessarily effective to inhibit polymerization of another diene. Chemistry is an empirical science and it is often difficult to predict in advance, without trying a particular experiment, whether a particular inhibitor or combination thereof will be successful or not. The subject invention herein is focused upon providing polymerization inhibitors for dienes, particularly 1,3-butadiene, also referred to simply as butadiene. Popcorn polymerization problems with butadiene will be described in more detail.
In the manufacturing of synthetic rubber, the primary choice of feedstock is butadiene. Butadiene (BD) is a colorless gas at room temperature. Most of the supply of butadiene comes from olefin plants because butadiene is co-produced when other olefins are manufactured. Butadiene can be produced by catalytically dehydrogenating butane or butylene.
The dehydrogenation of butane or butylene to butadiene may be accomplished by passing the feed gas over a catalyst bed at 1200xc2x0 F. (649xc2x0 C.) and at reduced pressure. The effluent gas then passes through an extractive distillation process.
Although butadiene can be manufactured by catalytic dehydrogenation, most of the butadiene produced domestically is obtained by extractive distillation. Butadiene can be recovered and purified from C4 streams by using a solvent that reduces the boiling point of the butadiene. The most popular solvents used to facilitate this extraction are N-methylpyrrolidone (NMIP) and dimethylformamide (DMF).
The crude butadiene is further purified by distillation through a series of towers that separate residual solvent, C4 compounds, and other contaminants. A further component of the BD manufacturing process is the Recovery Solvent Section. From the stripper column a slip stream of lean solvent is regenerated to remove heavies and contaminants. This reduces the presence of polymer loading and decomposition products.
During the purification steps of BD production, there are some undesirable BD polymerization reactions. Of these, the most unwanted is the formation of popcorn polymer. The effect of popcorn polymer on BD processing equipment is so severe that this material has actually been found to bend heat exchanger tubes.
Butadiene popcorn polymer is an insoluble but easily swelled polymer which varies in consistency. It has the unique property of generating more of the same type material in the presence of butadiene or another monomer.
As discussed previously, research investigations have shown that popcorn polymers grow by way of active free radical centers. These active centers are generated by the rupture of carbonxe2x80x94carbon bonds by strains resulting from swelling and growth. Although such phenomena have been observed both in BD plants and some research laboratories, there does not appear to be a standard test method that can be used to study butadiene popcorn polymer formation under laboratory controlled conditions.
The literature has suggested that some chemical additives have been found to be somewhat effective in controlling popcorn polymer formation. Chemical additives such as tert-butylcatechol (TBC), dinitro-tetraoxide, N,N-diethylhydroxylamine, hydroxybenzylphenylamines, and esters of organic sulfates are some of the additives that have been shown to negatively affect popcorn growth.
Accordingly, it is an object of the present invention to provide a method and composition to effectively inhibit the polymerization of diene compounds, in particular butadiene.
It is another object of the present invention to provide a method and composition to effectively inhibit the polymerization of dienes, such as butadiene, that is less expensive than using stable free radicals exclusively.
In carrying out these and other objects of the invention, there is provided, in one form, a composition for inhibiting polymerization of diene compounds which has at least two components. One component is a hindered or unhindered phenol which selected from the group of tert-butylcatechol (TBC); tert-butyl hydroquinone (TBHQ); 2,6-di-tert-butyl-4-methoxyphenol (DTBMP); 2,4-di-tert-butylphenol; 2,5-di-tert-butylphenol; 2,6-di-tert-butylphenol; 2,4,6-tri-tert-butylphenol; butylated hydroxyltoluene (BHT); 2,6-di-tert-butyl-4-nonylphenol; 2,6-di-tert-butyl-4-sec-butylphenol; 2-butyl-4-methylphenol; 2-tert-butyl-4-methoxyphenol; butylated hydroxyanisole (BHA); 2,5-di-tert-butyl hydroquinone (DTBHQ); tert-amyl hydroquinone; 2,5-di-amyl hydroquinone; 3,5-di-tert-butylcatechol; hydroquinone; hydroquinone monomethyl ether; hydroquinone monoethyl ether; hydroquinone monobenzyl ether; or 3,3,3xe2x80x2,3xe2x80x2-tetramethyl,1,1-spirobis-indane-5,5xe2x80x2,6,6xe2x80x2-tetrol (Tetrol). The second component is selected from the group of low nitrogen content components which may be a stable nitroxide and/or a hydroxylamine substituted with at least one alkyl, aryl or alkylaryl group, and/or a second hindered or unhindered phenol selected from the above group of hindered or unhindered phenols.